iOS gcd - 为什么不死锁？

Question

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"begin async:%@", [NSThread currentThread]);
        dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"in sync:%@", [NSThread currentThread]);
        });
        NSLog(@"end async:%@", [NSThread currentThread]);
    });
    NSLog(@"end main");
}

为什么我会得到如下结果？

1. end main
2. 开始异步：{number = 2，name =（null）}
3. 同步：{number = 2，name =（null）}
4. 结束异步：{number = 2，name =（null）}

正如苹果开发者文件所说：

  

将一个块提交到调度队列以进行同步执行。与dispatch_async不同，此函数在块完成之前不会返回。调用此函数并以当前队列为目标会导致死锁。

＆＃34;同步＆＃34;和＃34;结束异步＆＃34;永远不会退出，这是正确的吗？

Answer 1

但试试这个，如果你想看到死锁：

@interface ViewController ()
@property dispatch_queue_t q;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.q = dispatch_queue_create("myq", nil);
    dispatch_sync(self.q, ^{
        NSLog(@"begin sync1:%@", [NSThread currentThread]);
        dispatch_sync(self.q, ^{
            NSLog(@"in sync2:%@", [NSThread currentThread]);
        });
        NSLog(@"end sync1:%@", [NSThread currentThread]);
    });
    NSLog(@"end main");

}

@end